Pressure testing systems are used to test the integrity of devices, such as valves, prior to placing devices in service. In many industries, such as the oil and gas industry, certain devices must operate under very high pressures. Accordingly, corresponding pressure testing systems must test such devices under very high pressures. Historically, pressure testing systems have included safety features to restrict a device-under-test from becoming unclamped while pressurized, such as due to operator error. Such safety features historically were accomplished with pneumatic and/or hydraulic logic circuitry. FIG. 1 schematically illustrates such a prior art pressure testing system. In the illustrated prior art example, a device-under-test 10 (e.g., a valve) is clamped between two seal plates 12. Test pressure 14 is applied to the device-under-test 10 through one or both of the seal plates 12. The test pressure 14 is sensed by a pilot op switch 16, but because test pressures are typically too high for the pilot op switch 16, the pressure is reduced to signal level by a regulator 18 before it reaches the pilot op switch 16. The pilot op switch 16 is normally in a position that allows control air pressure 20 (e.g., shop air) to reach a pilot op valve 22, but when the device-under-test 10 is pressurized, the pilot op switch 16 moves to a position which blocks the control air pressure 20 from reaching the pilot op valve 22, and the pilot op switch 16 simultaneously vents downstream control air pressure from the pilot op valve 22. This moves the pilot op valve 22 to the closed position. Hydraulic clamping pressure 24 is applied past a check valve 26 to operate a hydraulic clamping cylinder 28 and can only be released through a manually operated clamp release valve 30 and the pilot op valve 22. When the pilot op valve 22 is in the closed position, hydraulic pressure cannot be released, even if the clamp release valve 30 is opened by the machine operator. In order to release the hydraulic clamping pressure 24, the test pressure 14 must be reduced to the switch threshold of the pilot op switch 16, and then the clamp release valve 30 will be effective.